Who is Bill Ackman? And can he do what he wants with Cincinnati-based Procter & Gamble Co.?

Ackman, 46, is a New York hedge fund manager who’s built a career, and a fortune, by being an activist investor. His net worth has been estimated at $2 billion.

Ackman typically buys enough shares in a company to use his standing to agitate for change – change he thinks will boost shareholder value.

His purchase Thursday of $2 billion in P&G (NYSE: PG) shares is the latest move. There have been many others, albeit not with companies as large as P&G, which has a $179 billion market cap.

Earlier this year, Ackman’s company, Pershing Square Capital Management, invested $1.4 billion for a 14 percent stake in Canada-Pacific Railway Ltd. He won a proxy fight, paving the way for him to pick a new CEO and nominate the company’s directors.

He built a big stake in J.C. Penney Co. Inc., becoming a director in 2011. He brought in the head of Apple Inc.’s retail division as CEO, to make big changes. Penney then drastically reduced discounts at its stores. Penney’s first quarter results were horrid; its stock price is down by almost half this year.

“He’s done this before with other companies and he has a broad respect in the industry as someone with some acumen,” said Greenberg, chair of the mergers and acquisitions practice at Cincinnati’s Taft Stettinius & Hollister law firm.

Greenberg said P&G’s board and management should meet with Ackman, but shouldn’t “let the one percent tail wag the $175 billion dog.” He thinks P&G will face increasing pressure to restructure, sell off slower-growing divisions and perhaps make management changes.

“The company’s already been under pressure. When you’re a growth stock and your stock price has been flat for six years, obviously you’re under pressure,” he said. “This ratchets it up a degree.”